Characterization of transport of caesium, strontium, cobalt and iron ions through concrete by steady-state migration and natural diffusion tests

Immobilization of wastes in cement-based matrices is being increasingly used, particularly in the case of elements of medium and low activity. Therefore, characterization of the system in relation to the release of ionic wastes through the cement, and consequently the determination of their diffusion coefficients, becomes of strategic importance. Nowadays, diffusion coefficients of some radioactive elements (mainly of Cs⁺) can be found in the literature, obtained by natural diffusion tests that are very time-consuming. In the present paper, the calculation of steady-state diffusion coefficients of Cs⁺, Sr²⁺, Co²⁺ and Fe³⁺ through concrete has been done by migration experiments. The mutual influence of the diffusion coefficients has also been studied, and transference numbers of the different ions have been calculated. The results have been compared with those obtained by natural diffusion tests, having found for Cs⁺ and Sr²⁺ that there is a very good accordance between diffusion and migration tests. In contrast for Co²⁺ and Fe³⁺ the accordance is not so good, because at migration working conditions they are able to form complex compounds. In consequence, migration tests are reliable for obtaining diffusion coefficients of different elements through concrete, always taking into account the nature of the species that are present at working conditions.